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Here are a few of the test helmet stamp marks made by the Crosby Manufacturing Company. The Company had their own Steel Lot stamp marks ie- KCco, NCco,and OCco, these were used to identify the different heats of steel made. Other later marks known to have been used by Crosby were WA, and ZAO, the company was involved in early Experimental and devised their own letter codes to identify the different heats of steel produced. Once M1917 production began to speed up, they asked if they could keep their own system of marking, but were told by the Ordnance Department that they would have to adopt the same system as the other companies. They were allocated the letter A by the Ordnance Department to identify their company on helmets. The ZAO mark is thought to be the last of the experimental lots marks used by this company before ZA1 started. Here are photo's of some of the marks from other sources. I would like to thank the author for this information found on another forum.